1. Field of the Invention
The present invention relates to a diamond single crystal substrate, and more particularly to a large, high-quality diamond single crystal substrate suitable for semiconductor materials, electronic components, and optical components or the like.
2. Description of the Prior Art
Diamond, like no other material, has a large number of excellent characteristics as a semiconductor material, those characteristics including high thermal conductivity, high electron and hole mobility, high insulation breakdown field, low dielectric loss, and a wide band gap. In particular, UV light-emitting devices using a wide band gap of diamond and field-effect transistors having excellent high-frequency characteristic. etc. have been developed in recent years. Moreover, because diamond is transparent within a range from ultraviolet (UV) to infrared (IR) regions, it is also promising as an optical component material.
In order to use diamond as a semiconductor, similarly to other semiconductor materials, large, high-quality single crystal substrates are required. Presently, diamond single crystals are industrially obtained mainly by a high-temperature high-pressure synthesis method. The crystallinity of the single crystals thus obtained is superior to that of naturally produced single crystals, but the size is difficult to increase above 10 mm in diameter and nitrogen is present as an impurity in the crystals unless special growth conditions are employed. The nitrogen-containing substrates are difficult to use directly as semiconductor single crystals. Moreover, due to absorption inherent to nitrogen, such substrates are not suitable as window materials for UV rays. Accordingly, an attempt was made to obtain a large, high-purity single crystal substrate by using the aforementioned substrate as a seed substrate for vapor phase growth and to conduct homoepitaxial growth (for example, Japanese Patent Publication Nos. 3-75298A and 2003-277183A).
When a diamond single crystal was homoepitaxially grown on a diamond single crystal seed substrate obtained by high-pressure synthesis, a phenomenon of residual stress accumulation in the vapor-phase grown layer was confirmed (for example, Japanese Patent Publication No. 2003-277183A). When a diamond single crystal substrate is obtained by forming a single crystal thick film by vapor phase growth, a problem of substrate cracking due to stress accumulation is encountered. Because the probability of cracking increases with the increase in substrate size (increase in surface area and thickness), this problem is not resolved even when a method described in Japanese Patent Publication No. 3-75298A is used, this method comprising the steps of forming a substrate serving as a nucleus for vapor phase growth by disposing a plurality of substances with a high-pressure phase that have substantially identical crystal orientation, growing a single crystal on this substrate by a vapor phase growth method, and obtaining an integrated large single crystal. Furthermore, the problems encountered during thick film growth are substantially not resolved even by conducting vapor phase growth of diamond single crystals from a seed substrate with a thickness of 100 μm or less, as described in Japanese Patent Publication No. 2003-277183A.